Installation:
(This is written on the premise you know how to get to your minecraft.jar)

CLIENT (minecraft.jar) -
1. Backup your minecraft.jar.
2. Delete META-INF.
3. Install ModLoader.
4. Drag the files in the "minecraft" folder into your minecraft.jar
5. Copy "HiddenDoors.props" into your /.minecraft/ folder.*
5. Start up minecraft and play.

* = HiddenDoors.props should generate anyway.